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  Resistor-string digital/analog converter circuit and semiconductor device including the same circuitUSPTO Application #: 20070052569Title: Resistor-string digital/analog converter circuit and semiconductor device including the same circuit Abstract: A resistor-string digital/analog converter circuit includes a plurality of resistors configured to divide a predetermined voltage, a plurality of MOS transistors configured to divide a voltage generated across one resistor of the plurality of resistors, a control circuit configured to control a connection between the one resistor and the plurality of MOS transistors so as to supply to the plurality of MOS transistors the voltage generated across the one resistor, and a gate potential generating circuit. The gate potential generating circuit generates a plurality of mutually different gate potentials and supplies the plurality of generated gate voltages, respectively, to a plurality of gates of the plurality of MOS transistors, so that the resistance values of MOS resistance of the plurality of MOS transistors are equal to each other. (end of abstract) Agent: Banner & Witcoff, Ltd., Attorneys For Reserve Attorneys For Client No. 000449, 001701 - Washington, DC, US Inventors: Asami Saito, Satoshi Sakurai USPTO Applicaton #: 20070052569 - Class: 341154000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20070052569. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-256822 filed on Sep. 5, 2005; the entire contents of which are incorporated herein by reference. BACKGROUND OF THE INTENTION [0002] 1. Field of the Invention [0003] The present invention relates to a resistor-string digital/analog converter circuit and a semiconductor device including the same circuit, and more particularly to a resistor-string digital/analog converter circuit using MOS resistance as interpolation resistance, and a semiconductor device including the same circuit. [0004] 2. Description of Related Art [0005] As one sort of digital/analog converter circuit (hereinafter, "D/A" for short), there has hitherto been known a resistor-string D/A converter circuit (R-DAC). In the resistor-string D/A converter circuit, a plurality of resistors connected in series are used and an analog voltage is extracted from a voltage dividing point corresponding to an input digital signal, whereby D/A conversion is performed. Generally, in the resistor-string digital/analog converter circuit, resolution is sometimes improved using interpolation resistance. [0006] There has been proposed a technique by which, when a plurality of MOS resistances are used as the interpolation resistance, in order to reduce the variation in MOS resistance value ascribable to body effect, a source potential of one MOS transistor among the interpolation resistances is monitored to vary the gate potential of each MOS transistor (for example, refer to U.S. Pat. No. 5,943,000). [0007] More specifically, the source potential (V.sub.pol) of one MOS transistor of the interpolation resistances are monitored and the gate potential (V.sub.gate) created based on the source potential (V.sub.pol) monitored is supplied to the gate of all the MOS transistors. Accordingly, the technique according to the proposal reduces the variation in MOS resistance value caused by a variation in position to which the MOS transistors are connected, thereby improving linearity of D/A conversion. [0008] However, in the technique according to the proposal, the source potential differs between each MOS transistor, and MOS resistance value is in inverse proportion to the difference between gate-source voltage (V.sub.gs) and threshold voltage (V.sub.th). Therefore, the MOS resistance values of the plurality of MOS transistors are not equal to each other, thus causing a problem. [0009] That is, even when the same potential is applied to the gates of all the MOS transistors, since the source potential of each MOS transistor is different, there arises a difference in gate-source voltage (V.sub.gs), causing nonuniformity of MOS resistance values. Consequently, satisfactory linearity of D/A conversion cannot be achieved. Particularly, when a large voltage is applied to the plurality of MOS transistors, the nonuniformity is noticeable. BRIEF SUMMARY OF THE INVENTION [0010] According to one aspect of the present invention, there is provided a resistor-string digital/analog converter circuit converting a digital signal into an analog signal, the circuit including: a plurality of resistors configured to divide a predetermined voltage; a plurality of MOS transistors configured to divide a voltage generated across one resistor of the plurality of resistors; a control circuit configured to control a connection between the one resistor and the plurality of MOS transistors so as to supply to the plurality of MOS transistors the voltage generated across the one resistor; and a gate potential generating circuit configured to generate a plurality of mutually different gate potentials and supply the plurality of generated gate voltages, respectively, to a plurality of gates of the plurality of MOS transistors so as to make the resistance values of MOS resistance of the plurality of MOS transistors equal to each other. [0011] According to one aspect of the present invention, there is provided a semiconductor device including a resistor-string digital/analog converter circuit converting a digital signal into an analog signal, the semiconductor device including: a plurality of resistors configured to divide a predetermined voltage; a plurality of MOS transistors configured to divide a voltage generated across one resistor of the plurality of resistors; a control circuit configured to control a connection between the one resistor and the plurality of MOS transistors so as to supply to the plurality of MOS transistors the voltage generated across the one resistor; and a gate potential generating circuit configured to generate a plurality of mutually different gate potentials and supply the plurality of generated gate voltages, respectively, to a plurality of gates of the plurality of MOS transistors so as to make the resistance values of MOS resistance of the plurality of MOS transistors equal to each other. [0012] According to one aspect of the present invention, there is provided a semiconductor device including a resistor-string digital/analog converter circuit converting a digital signal into an analog signal, the semiconductor device including: an input unit configured to receive the digital signal; a plurality of resistors configured to divide a predetermined voltage; a plurality of MOS transistors configured to divide a voltage generated across one resistor of the plurality of resistors; a gate potential generating circuit configured to generate a plurality of mutually different gate potentials and supply the plurality of generated gate voltages, respectively, to a plurality of gates of the plurality of MOS transistors so as to make the resistance values of MOS resistance of the plurality of MOS transistors equal to each other; a control logic circuit configured to control a connection between the one resistor and the plurality of MOS transistors so as to supply the voltage generated across the one resistor to the plurality of MOS transistors by selecting, according to higher-order bits of the digital signal, the one resistor and connecting both ends of the one resistor selected, respectively, to the plurality of MOS transistors; and an output unit configured to output as the analog signal a source voltage or a drain voltage of one MOS transistor of the plurality of MOS transistors which is selected according to lower-order bits of the digital signal. BRIEF DESCRIPTION OF THE DRAWINGS [0013] FIG. 1 is a circuit diagram showing a configuration of a resistor-string D/A converter circuit according to a first embodiment of the present invention; [0014] FIG. 2 is a circuit diagram showing an exemplary gate potential generating circuit according to the first embodiment; [0015] FIG. 3 is a partial circuit diagram of a resistor-string D/A converter circuit according to a second embodiment of the present invention; [0016] FIG. 4 is a circuit diagram of a gate potential generating circuit according to the second embodiment of the present invention; [0017] FIG. 5 is a partial circuit diagram of a resistor-string D/A converter circuit according to a third embodiment of the present invention; [0018] FIG. 6 is a circuit diagram of a gate potential generating circuit according to the third embodiment; and [0019] FIG. 7 is a circuit diagram showing a variation of the gate potential generating circuit according to the third embodiment. 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